Attention is being given to FBARs, which are resonators using the thickness longitudinal vibration of a piezoelectric material, as filter elements for high-frequency communication.
FIG. 23 is a plan view of an FBAR. FIG. 24 is a cross-sectional view taken along line A-A in FIG. 23. As illustrated in FIGS. 23 and 24, the FBAR has a structure in which a piezoelectric film 103 is sandwiched by an upper electrode 101 and a lower electrode 102, and a region defined by the upper electrode 101 and the lower electrode 102 which are opposed to each other acts as an actual resonator (hereinafter referred to as the resonant portion R1). Provision of a cavity 105 or an acoustic reflector above and below the resonant portion R1 may prevent attenuation of elastic waves generated at the resonant portion R1 and provide a resonance characteristic with a high quality factor (Q).
As illustrated in FIG. 25, piezoelectric thin-film resonators have a vibration mode having a transversely propagating component (transverse mode) in a frequency band near the resonance frequency and the antiresonance frequency of the thickness vibration. A transverse-mode wave W1 is reflected by an edge of the resonant portion R1 (reflected wave W2) or passes through the edge of the resonant portion R1 to propagate to a nonresonant portion R2. The wave propagating to the nonresonant portion R2 will be lost (for example, if the reflection characteristic is represented by a Smith chart, a Q circle appears as a small circle). In an actual resonator structure, an edge of each of the upper electrode 101 and lower electrode 102 is sloped as illustrated in FIGS. 26 and 27A for convenience of manufacturing and the apparent acoustic impedance of the piezoelectric film 103 gradually decreases toward the nonresonant portion R2 as illustrated in FIG. 27B. (Apparent acoustic impedance Z exists as opposed to intrinsic acoustic impedance which would exist in the absence of a mass element, because acoustic impedance Z is equal to the density ρ of a material multiplied by speed of sound c and the speed of sound changes when a mass element is added to the piezoelectric film. Herein, the acoustic impedance is defined as being equal to the apparent acoustic impedance.)
In theory, in a resonator using a piezoelectric film having a Poisson ratio of 1/3 or less, transverse-mode waves tend to pass through the edge and be lost if the acoustic impedance of the portion surrounding the resonant portion is smaller than that of the resonant portion. Actual optical observations show that leaked waves exist outside the resonator. Therefore there is the problem of preventing the leaked waves.
Examples of conventional techniques for preventing leakage of transverse waves include the techniques disclosed in JP2003-505906 and JP2006-109472. These Japanese Laid-open Patent Publications disclose configurations in which the perimeter of a resonant portion R1 is uniformly enclosed by a layer 106 having a different acoustic characteristic as illustrated in FIGS. 28 and 29. The configurations may have the effect of preventing leakage of transverse waves as theoretically predicted because the acoustic impedance of the perimeter of the resonator is greater than the acoustic impedance of an exciting portion.